This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We wish to implement various theoretical methods for the study of fragmentation processes of various classes of biomolecules to complement experimental data, based of tandem MS/MS methods with isotopic labeling and model compounds. We are approaching these problems by using density functional and ab initio calculations as implemented in the Gaussian 03/98 suites of programs for Linux. We have these programs running on two dual/dual-core (quad-processor) installed in this project period and three dual-processor workstation computers and a single-processor workstation computer installed during the previous project period. In addition, Spartan for Linux was installed on the last computer and serves roles in preliminary survey calculations and as a graphical interface. These current resources facilitate the tackling of larger compounds and more complex processes e.g. unusual generation of radical cations from even-electron precursors. One focus involves the fragmentation of modified nucleobases, adducted by polycyclic aromatics hydrocarbons, modified endogenous steroids, and phytosterols;this general area is important for developing analytical methods for cancer biomarkers. Another focus grows from the long-term interest for the Washington University resource in the structural determination of various lipids including phospholipids. In addition, current experimental studies aimed at formation and identification of sterol-modified nucleobases are being pursued at the resource. In all of these studies, the mechanisms of fragmentation need to be understood to provide a firm foundation for structural studies.